Blocking is a term given to describe sticking between two or more layers of coated substrate, which may be printed, or unprinted, and in roll, sheet, or package form.
Another term to be considered is setoff, which describes the smearing or the transferring of ink from freshly printed material to another surface with which there is contact to the undried print.
Aqueous coatings, comprised of mixtures of aqueous emulsions, were developed to improve productivity by allowing printed/coated substrates to be handled, while underlying inks are drying. In addition, aqueous coatings are specifically formulated to provide a wide variety of highly desirable physical and aesthetic properties.
Aqueous emulsion coatings dry by the evaporation of volatiles, mostly water, leaving coating solids to coalesce, or grow together, to form a flexible, formable barrier film. Another factor to consider is the physical nature of the dry aqueous coating film, which may be described as micro porous. This porosity or breathability is required so that oxidizing sheet-fed offset inks are exposed to the oxygen in air, which combines with the ink vehicle to produce a dry ink film. The oxidation process, being time dependent, takes time to complete. The fact that the underlying oxidizing inks require hours, if not days to dry, has led to setoff failures. This has been seen, especially over heavy ink coverage, where ink has been extruded through the coating film and transferred to an adjacent sheet.
Aqueous coatings have seen continuous volume growth since they first appeared during the 1970’s. Various formulations have been marketed by a number of suppliers, and there has been tremendous success with aqueous coatings being applied on a variety of application equipment. These coatings have been used over a wide variety of printing inks, and just as wide a variety of substrates. In any analysis, all are a factor in how aqueous coatings perform.
Thin, dry to the touch, aqueous coated surfaces are vulnerable to blocking, and/or sticking, when exposed to enough heat, pressure, and humidity. This can happen, especially in a load of printed/coated sheets, during a sheetfed offset printing, and coating process.
So, what can be done to prevent blocking?
First of all, be aware that aqueous coatings, based on aqueous emulsions, are subject to being molded, or shaped by a combination of heat, pressure, and humidity. These coatings, depending on the formulator’s choice of polymers and other raw materials, will have a blocking temperature that can vary, and therefore some aqueous coatings will block at lower temperatures than others. Since you alone can know the performance requirements of your customer, you must understand what the limits of your aqueous coated production must be.
What can you do?
There are several things that you alone can do. You can understand the limitations of your equipment, and your processes. You can know the performance requirements of your customers. You can request coatings from your supplier to meet the objectives of both you and your customers. If necessary, you can request an aqueous coating with a higher blocking resistance. Some suppliers routinely supply coatings with a higher blocking resistance. Typically, higher gloss coatings are softer, offer less blocking resistance in terms of the polymer section, and use minimal additions of slip agents that normally would improve blocking resistance.
You can also be aware that thicker applications of coatings are more difficult to dry. Substrates such as poly board, and foil and film laminated board create more holdout, causing all of the coating to be on the surface, effectively a thicker application that is slower to dry. Coating applications over areas of very heavy ink coverage cause a similar effect. Any build-up of coating on the side edges, or the lead, or trailing edge of a substrate is also to be avoided.
If you are coating both sides of a substrate or “Work & Turning”, it is recommended that you print/coat the first side with pile temperatures between 85° and 95° F. Too much I-R heat retained in the pile can also cause blocking and even setoff through the coating. When “Work & Turning”, it is strongly advised that the pile temperature be no higher than 90° F. before the second side coating pass is attempted. A light application of small micron particle size spray power will also be helpful, improving blocking and setoff resistance, especially when using high gloss coatings. Turn the I-R units down or even off, and maximize the hot air flow, when running the second side so that the pile temperature does not exceed 85-88° F. You may also have to reduce press speed, reduce pile size, or air a load.
Remember, all aqueous emulsion coatings block at some combination of temperature, pressure, and humidity. Reduce these in production, in inventory, and in shipping. It will pay you to know where the limits are! Ask, test and know, in order to avoid blocking.
Contact Cork Technical experts if you require an aqueous coating with enhanced performance properties for a demanding application.
Corks’ business is the development and formulation of Aqueous, energy-curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives.